Two populations, each of 40 wild x domestic 6-way crosses of rats (Rattus norvegicus), were studied over an 18-month period in interconnected 33-compartment habitats. Passive resonators implanted under the abdominal skin permitted computer recording of the identity of each rat each time it passed through "portals" connecting adjoining compartments. Time, place, and direction of travel were simultaneously recorded. Males aggregate in the vicinity of compartments between which estrous females move. A "Close Space-Time Association" (CSTA) index was developed to calculate the degree of male-female associations. On days of estrus there is one hour of extremely heightened activity by the female. The CSTA index, during the hour of her heightened activity, is the sum of the number of males which enter a compartment within 5 seconds before, or 5 seconds after, the time that the female entered it. This index is of the order of 45x that of comparable hours on non-estrous days. Calculating this index for hours of high activity each day now makes it possible to plot the course of estrus over the lifetime of all females in the populations. Since group dynamics are markedly modified by estrus it will now be possible to contrast data between estrous and non-estrous days.